The Java EE 5 Tutorial

The Life Cycles of Enterprise Beans

An enterprise bean goes through various stages during its lifetime, or life cycle.
Each type of enterprise bean (stateful session, stateless session, or message-driven) has a
different life cycle.

The descriptions that follow refer to methods that are explained along with the
code examples in the next two chapters. If you are new to
enterprise beans, you should skip this section and run the code examples first.

The Life Cycle of a Stateful Session Bean

Figure 20-3 illustrates the stages that a session bean passes through during its lifetime.
The client initiates the life cycle by obtaining a reference to a stateful
session bean. The container performs any dependency injection and then invokes the method annotated
with @PostConstruct, if any. The bean is now ready to have its business
methods invoked by the client.

Figure 20-3 Life Cycle of a Stateful Session Bean

While in the ready stage, the EJB container may decide to deactivate, or
passivate, the bean by moving it from memory to secondary storage. (Typically, the
EJB container uses a least-recently-used algorithm to select a bean for passivation.) The
EJB container invokes the method annotated @PrePassivate, if any, immediately before passivating it. If
a client invokes a business method on the bean while it is in
the passive stage, the EJB container activates the bean, calls the method annotated
@PostActivate, if any, and then moves it to the ready stage.

At the end of the life cycle, the client invokes a method
annotated @Remove, and the EJB container calls the method annotated @PreDestroy, if
any. The bean’s instance is then ready for garbage collection.

Your code controls the invocation of only one life-cycle method: the method annotated
@Remove. All other methods in Figure 20-3 are invoked by the EJB container. See
Chapter 34, Resource Connections for more information.

The Life Cycle of a Stateless Session Bean

Because a stateless session bean is never passivated, its life cycle has only
two stages: nonexistent and ready for the invocation of business methods. Figure 20-4 illustrates the
stages of a stateless session bean.

Figure 20-4 Life Cycle of a Stateless Session Bean

The client initiates the life cycle by obtaining a reference to a
stateless session bean. The container performs any dependency injection and then invokes the method
annotated @PostConstruct, if any. The bean is now ready to have its business
methods invoked by the client.

At the end of the life cycle, the EJB container calls the
method annotated @PreDestroy, if any. The bean’s instance is then ready for garbage
collection.

The Life Cycle of a Message-Driven Bean

Figure 20-5 illustrates the stages in the life cycle of a message-driven bean.

Figure 20-5 Life Cycle of a Message-Driven Bean

The EJB container usually creates a pool of message-driven bean instances. For each
instance, the EJB container performs these tasks:

If the message-driven bean uses dependency injection, the container injects these references before instantiating the instance.

The container calls the method annotated @PostConstruct, if any.

Like a stateless session bean, a message-driven bean is never passivated, and it
has only two states: nonexistent and ready to receive messages.

At the end of the life cycle, the container calls the method
annotated @PreDestroy, if any. The bean’s instance is then ready for garbage collection.